1. Field of the Invention
The present invention relates to a dielectric filter of narrow band width used in communication apparatuses, for example, an automobile telephone.
2. Description of the Prior Art
In a high-frequency filter of narrow band width in mobile radio communication system such as an automobile telephone being a typical example, a dielectric filter satisfying requirements of small size, stoutness and high performance is generally used. Such dielectric filter is usually constituted in a filter of comb line type so as to reduce unit cost of the products.
A dielectric filter of comb line type in the prior art will be described referring to FIG. 5 and FIG. 6. Numeral 1 designates a dielectric block, numeral 2 a hole, numeral 3 an inner conductor, numeral 4 an earth electrode, numeral 5 a short-circuit electrode, numeral 6 an input or output terminal, and numeral 7 a terminal base to hold the terminal 6. The dielectric block 1 is made by sintering of dielectric powders, and a plurality of holes 2 are provided on the dielectric block 1, and the inner conductor 3 is formed inside each hole 2. The earth electrode 4 is formed on the lateral surface outside the dielectric block 1, and the short-circuit electrode 5 to connect the inner conductor 3 with the earth electrode 4 is formed on one end surface of the dielectric block 1. The terminal base 7 is inserted in the hole 2 provided on the dielectric block 1 so as to support the input or output terminal 6 and to form a capacitor (capacitors 13, 14 in FIG. 9) between the terminal 6 and the inner conductor 3, and the terminal 6 is inserted in the hole of the terminal base thereby the dielectric filter is constituted. In addition, dielectric substance of the inner conductor 3, the earth electrode 4 and the short-circuit electrode 5 is formed by applying silver paste and burning it or by means of electroless copper plating.
Operation principle of a comb line filter in such constitution is disclosed in references, for example, G. L. Matthaei, "Comb-Line Band-Pass Filters of Narrow or Moderate Band Width" The Microwave Journal vol. VI, No. 8 (August 1963). Consequently, the detailed description shall be omitted but portion relevant to the present invention be described here.
FIG. 9 shows an equivalent circuit of a comb-line filter constituted in two steps. Numerals 11, 12 designate distributed constant lines, numerals 13, 14 coupling capacitors for impedance conversion, numerals 15, 16 terminating capacitors, and numerals 17, 18 input and output terminals. The distributed constant lines 11, 12 are coupled electromagnetically with each other, and length of the distributed constant lines 11, 12 is less than the 1/4 wavelength of the center frequency of the filter passing frequency band (the center frequency: arithmetic mean value of the upper limited frequency and the lower cut-off frequency of the passing signal). One end of the distributed constant lines 11, 12 is directly grounded, and other end thereof is grounded through the terminating capacitors 15, 16. The terminating capacitors 15, 16 are small capacitance produced when electric lines of force between the inner conductor 3 and the earth electrode 4 shown in FIGS. 5 and 6 slightly leak in the space at the open end of the dielectric filter (at side of the terminal 6), i.e., so-called fringing capacitance. If the capacitance value of the terminating capacitors 15, 16 is large, the distributed constant lines 11, 12 are shortened; if the capacitance value is small, the lines 11, 12 are lengthened. In any case, however, the length of the distributed constant lines 11, 12 is less than the 1/4 wavelength.
In order to use such dielectric filter in the prior art, an earth plate 19 is required as shown in FIGS. 7 and 8. The earth plate 19 is closely contacted with the dielectric filter, i.e., with the earth electrode 4 and in electrically connected (conductive) state. The earth plate 19 is used to mount and hold the dielectric filter on a substrate or a chassis, for example, and a mounting member 20 is installed therefor.
Condition to secure the operation of the dielectric filter is in that the earth electrode 4 of the dielectric filter is securely grounded to the external circuit, for example, to the earth pattern of the substrate or the chassis. Consequently, the earth plate 19 is securely connected to the earth electrode 4 by means of a conductive adhesive or soldering, and at the same time grounded securely to the external circuit.
In the dielectric filter in the prior art as above described, since the terminating capacitors 15, 16 are constituted by the capacitance produced when the electric lines of force leak from the open end of the dielectric filter, the capacitance is small and the distributed constant lines 11, 12 constituted by the inner conductor 3 are lengthened and therefore the dielectric filter cannot be made compact. Moreover, since the earth plate 19 is used so as to ground the earth electrode 4 of the dielectric filter securely, work process such as soldering or applying of a conductive adhesive is required to connect the earth electrode 4 with the earth plate 19 securely. The connection by soldering or the like may become insecure and cause the incomplete grounding, thereby the dielectric filter is liable to the unstable operation.